1. Field of the Invention
This invention relates to a recording sheet, and more particularly to a recording sheet using an improved developer.
2. Description of the Prior Art
A recording sheet which makes use of a coloring reaction between a substantially colorless organic compound (hereinafter designated "coloring agent" ) and an absorbent or reacting compound (hereinafter designated "developer" ) which is colored upon contact with the coloring agent is well known. Among the coloring agents, there are Malachite Green lactone, benzoyl leucomethylene blue, crystal violet lactone, Rhodamine B lactam, 3-dialkylamino-7-dialkylaminofluoran, 3-dialkylamino-7-N-aryl N-alkylamino fluoran and 3-methyl-2, 2-spirobi(benzo [f] chromene).
As the recording sheet employing this coloring reaction, there are the pressure sensitive copying papers (for example as described in U.S. Pat. Nos. 2,505,470; 2,505,489; 2,550,471; 2,548,366; 2,712,507; 2,730,456; 2,730,457; and 3,418,250) and heat sensitive copying papers. (For example as described in Japanese Pat. No. 4160/67 and U.S. Pat. No. 2,939,009).
On the other hand, a printing process for obtaining a colored image comprising feeding an ink containing the coloring agent in a solvent through a stencil or the like on a sheet applied with the developer is described in German Pat. No. 1,939,624.
Usually, the coloring reaction between the developer and the coloring agent requires the pressure of a pen or typewriter, or another physical stimulus.
A pressure sensitive copying paper, a typical example of copying papers, is produced by dissolving the coloring agent in a solvent such as alkylated naphthalene, alkylated diphenyl or alkylated diphenylmethane, dispersing the solution in a binder or encapsulating the solution in microcapsules, and applying the solution thus treated on a sheet of paper, plastic, resin coated paper or a like support.
On the other hand, a heat sensitive copying paper is produced by applying the coloring agent on the support together with acetanilide, para-toluene-N, N-diethylsulfonamide or a like thermofusable material. A thermofusing material is a material which is fused by heat and dissolves the coloring agent.
The developer is dissolved or dispersed in water or an organic solvent together with a binder, and the resulting solution or dispersion is applied on or impregnated in the support. The developer can be applied on or impregnated in the support as the ink.
Generally, the coloring agent and the developer are applied on the same surface or mutually different surfaces of the support, or on mutually different supports.
Known developers are acidic terra alba, activated terra alba, attapulgite, zeolite, bentonite or like clays, succinic acid, tannic acid, gallic acid, phenol compounds or like organic compounds, and phenol resins or like acidic polymers. Especially, the phenol resin is given attention as a novel developer in Japanese Pat. No. 20144/67, and various improvements on phenol resin developers have been proposed, for example, in U.S. Pat. Nos. 3,516,845; 3,525,630; 3,540,911; and 3,634,121; British Pat. Nos. 1,065,587; and 1,215,618.
According to such prior art, the phenol resin is divided into fine particles and dispersed in water, or mixed with gum arabic in a ball mill and dispersed in water, or otherwise dissolved in an organic solvent.
The prior art, however, has the following defects: In using fine particles of the phenol resin, the preparation of the fine particles is difficult and requires much time. It is desirable that the particle size of the resin be small to obtain sufficient color density. However, the minimum particle size is restricted by grinding methods and a large amount of the ground resin must be used in order to obtain sufficient color density.
In a case of preparing the coated solution by mixing the phenol resin with gum arabic or water in a ball mill, the preparation of the coating solution requires much time and in addition the batch size of the coating solution is restricted by the working conditions. Accordingly, the preparation of the coating solution on a commercial scale is difficult.
In dissolving the phenol resin in a solvent, means for protecting against explosion must be attached to the apparatus in which the coating solution is prepared and to the coater, since the solvent is often poisonous and easily inflammable.
As described above, the prior art for the preparation of phenol resin containing coating solutions have various defects and do not utilize fully the developing property of the phenol resin.
Japanese Pat. No. 20971/72 and British Pat. No. 1,215,618 disclose that the developing ability of the developer sheet and the light resistance of the colored image on the developer sheet are improved by adding a chloride, nitrate, sulfate and acetate of a bivalent metal, such as cadmium, calcium, magnesium, barium, manganese, nickel or the like. In such case, however, both the developing ability of the phenol resin and the effect of the bivalent metal salt are not fully exhibited as the fine particles of phenol resin are used usually.
An object of this invention is to provide a novel recording sheet having improved developing ability.
Another object of this invention is to provide a recording sheet having a color image of improved light resistance.
A further object of this invention is to provide a process for producing a recording sheet in which the coating solution can be simply prepared.